1. Field of the Invention
The present invention relates to a multi-electron source, an image-forming device using the multi-electron source and methods for preparing them.
2. Related Background Art
As an element of a simple structure for emitting electrons, for example, a cold cathode element is heretofore known which has been reported by M. I. Elinson et al. [Radio Eng. Electron. Phys., Vol. 10, pp. 1290-1296 (1965)].
This kind of element utilizes the phenomenon that electrons are emitted by allowing current to flow in parallel to the surface of a thin film having a small area formed on a substrate, and it is usually called a surface conduction type electron emitting element.
As examples of this surface conduction type electron emitting element, there have been reported an element using an SnO.sub.2 (Sb) thin film developed by Elinson et al. as mentioned above, an element using an Au thin film [G. Dittmer, "Thin Solid Films", Vol. 9, pp. 317 (1972)], an element using an ITO thin film [M. Hartwell and C. G. Fonstad, IEEETrans. ED Conf., pp. 519 (1975)], and an element using a carbon thin film [Hisashi Araki et al., "Shinku (Vacuum)", Vol. 26, No. 1, pp. 22 (1983)].
FIG. 1 shows the constitution of a typical one of these surface conduction type electron emitting elements. In this drawing, reference numerals 1 and 2 are electrodes for giving an electrical connection, numeral 3 is a thin film made of an electron emitting material, 4 is a substrate, and 5 is an electron emitting portion (crack portion), and L is a width of the electron emitting portion.
Heretofore, in the surface conduction type electron emitting element, the electron emitting portion is formed by a resistive heating treatment called "forming" prior to carrying out electron emission. That is, voltage is applied between the electrodes 1 and 2 to electrify the thin film 3, so that Joule heat is generated and this heat locally breaks, deforms or modifies the thin film 3 to form the electron emitting portion 5 which is in an electrically highly resistant state, whereby an electron emitting function is obtained.
The above-mentioned "electrically highly resistant state" means a discontinuous state of the thin film 3 in which a crack having a width of 1.0 .mu.m to 5 .mu.m is partially formed and it has the so-called island structure. This thin film is physically discontinuous but electrically continuous.
In the case of the conventional surface conduction type electron emitting element, voltage is applied to the above-mentioned highly resistant discontinuous film through the electrodes 1 and 2 to electrify the surface of the element, whereby electrons can be emitted from the fine particles.
However, the electron emitting element prepared by the forming treatment using the conventional resistive heating technique has the following problems.
1) It is impossible to design the island structure of the electron emitting portion, and therefore the improvement of the element is difficult and the quality is also liable to be uneven among the elements.
2) Since a large Joule heat is generated in the forming step, the substrate tends to be broken, and for this reason, multiplication is difficult.
3) The material of the island is limited to gold, silver, SnO.sub.2, ITO and the like, and so a material having a small work function cannot be used. Thus, a large emitting current cannot be obtained.
In view of the above-mentioned points, the surface conduction type electron emitting element has not been positively utilized on an industrial scale, though it has the advantage that the element structure is simple.
The present inventors have intensively investigated to solve the above-mentioned problems, and as a result, in U.S. Pat. No. 5023110 (Japanese Patent Application Laid-open No. 2-56822), they have suggested a novel surface conduction type electron emitting element in which a fine particle film is disposed between electrodes and an electron emitting portion is formed by a conduction treatment (voltage applying treatment). A constitutional view of this novel electron emitting element is shown in FIG. 2.
In this drawing, numerals 11 and 12 are electrodes, 13 is a fine particle film, 14 is an electron emitting portion (crack portion), 15 is a substrate, and L is a width of the electron emitting portion.
Features of this electron emitting element are as follows:
1) Since the electron emitting portion 14 can be formed by allowing very small current to flow in the fine particle film 13, the element which is free from degradation can be prepared. In addition, the shape of the electrodes can be optionally designed.
2) The fine particles constituting the fine particle film are a constitutional material for the electron emission, and therefore, the selection of the fine particle material and the design of the fine particle shape are possible, which means that electron emission properties can be designed.
3) Materials of the substrate 15 end the electrodes which are constitutional members of the element can be selected from a wide range.
Examples of practical articles of the electron emitting element described above include various electron beam application equipments such as displays, fluorescent lamps, ion generators, etc. In recent years, a device using a plate electron source in which such elements are multiply arranged, for example, a flat CRT shown in Japanese Patent Application Laid-open No. 61-221783, has been energetically researched end developed.
Now, in order to prepare a plate electron source in which surface conduction type electron emitting elements are multiply arranged, it is usually necessary to take such an element arrangement as shown in FIG. 3.
In this drawing, reference numeral 21 is a substrate, numeral 24 is an electron emitting element comprising element electrodes 22 end an electron emitting portion 23, 25 is a wiring electrode, 26 is a power source for forming, end 27 is a connection wire for electrically connecting the wiring electrode 25 to the power source 26. In this drawing, the electron emitting portion 23 corresponds to the electron emitting portion 5 in FIG. 1 or the electron emitting portion 14 and the fine particle film 13 in FIG. 2.
For the preparation of the plate electron source using such surface conduction type electron emitting elements, it is necessary to arrange a plurality of the electron emitting elements 24 between the wiring electrodes 25 as in FIG. 3 and to further carry out an overall forming treatment to the plurality of electron emitting element.
However, in the case that a plurality of electron emitting portions as in FIG. 3 are formed at a time by using the conventional forming treatment in which a DC voltage is very slowly applied (e.g., at a voltage rise rate of 1 volt/minute) in a vacuum, the following drawbacks are present.
(1) In the overall forming treatment of a plurality of fine particle films as shown in FIG. 2, the temperature rise at the time of the forming is significantly, which leads to the degradation of properties and renders characteristics of the respective elements ununiformed.
(2) In the overall forming treatment of a plurality of conductive thin films as shown in FIG. 1, a still larger amount of heat is generated at the time of the forming, and therefore the problem of the breakage of the substrate and the element electrodes are raised in addition to the problem in the above-mentioned paragraph (1).
(3) Additionally, in order to uniformly emit a large number of electron beams from the plate electron source, it is necessary to arrange the electron emitting elements 24 in the state of high density, and in this case, the drawbacks in the preceding paragraphs (1) and (2) are emphasized.
Next, reference will be made to an image.sup.u forming device shown in FIG. 8 in which a plurality of the above-mentioned electron emitting elements are arranged. In FIG. 8, numeral 21 is an insulating substrate (a rear plate), 25 and 26 are wiring electrodes, 31 is a modulation means (a grid electrode), 32 is an electron passage orifice, 41 is a rear plate, 42 is an element wire, 43 is a grid electrode wire, 44 is a transparent electrode, 45 is a fluorescent member, 46 is a glass plate, 47 is a face plate consisting of the members 44, 45 and 46, and 48 is an EV terminal. The interior of such an image-forming device is kept under a vacuum state by the rear plate 41, the face plate 47 and the like, as shown in the same drawing.
In the image-forming device (flat CRT) described above, voltage based on an information signal is applied to the element wires 42 and grid wires 43 (the element wires 42 are connected to the wiring electrodes 25 and 26, and the grid wires 43 are connected to the grid electrodes 31), and electrons emitted from the electron emitting elements 24 are ON/OFF-controlled by the grid electrodes 31 to allow the electrons to collide against the fluorescent member 45, whereby a predetermined image is displayed.
In such an image-forming device, the above-mentioned drawbacks of the multi-electron source which take place in the forming step for forming a plurality of electron emitting portions give rise to fatal problems such as defective display and uneven display.